The present invention relates to an infinitely variable cone-pulley transmission having a pair of cone pulley sheaves arranged on the drive-input shaft and a pair of cone pulley sheaves arranged on the drive-output shaft and transmission means circulating between the two pulley sheave pairs.
In transmissions of the type to which the invention is directed, one of the cone pulley sheaves on each shaft is axially displaceable along that shaft but is connected, at least indirectly, in rotation with the shaft and is formed as the axially displaceable pressure cylinder, rotating with the shaft, of a piston fixed to the shaft. A pressure medium for the setting and maintenance of the transmission ratio is distributed to the cylinder by a control slide valve which has four control edges and which is connected via a setting lever with one of the axially displaceable cone pulley sheaves. The two other cone pulley sheaves are supported so as to be axially fixed relative to the shaft.
For the load-dependent generation of pressure-application forces at least on one transmission shaft there is provided a sensor for the torque on that shaft, to which sensor the pressure medium flows under the pressure prevailing on its own or the opposite shaft in the cylinder-piston unit thereof for the setting and maintenance of the transmission ratio. This sensor generates by a torque-dependent movement of at least two valve parts of a valve in relation to one another, a torque-dependent hydraulic pressure and exclusively determines the requisite load-dependent, hydraulic pressure-application force.
Regarding the above-mentioned classification of transmissions, it must be remarked firstly that in the mentioned cylinder-piston unit it is of course also possible to effect an interchange of function by forming the cone pulley sheave which is displaceable for the transmission ratio as a piston which is displaceable in a cylinder fixed to the shaft. Ordinarily, however, the initially mentioned construction is used, for which reason it will be adopted for the following description of the invention.
The initially mentioned hydraulically adjustable cone-pulley transmissions are disclosed, by way of example, in U.S. Pat. No. 3,600,960. Beside their inherently very good properties in use however they have some disadvantageous properties which hinder their use for special or more demanding needs.
Thus, firstly, their construction costs are considerable, irrespective of whether the torque sensor is integrated into a pulley assembly or whether, as appears from an as yet undisclosed proposal, it is formed as an externally arranged prefabricated unit. The construction expense affects not only the formation of the torque sensor per se, but also its space requirement which has an unfavorable effect upon the overall size of the transmission in each case.
Furthermore, it is disadvantageous that on the occurrence of torque surges, as in starting up of the transmission, in the shifting of couplings of the driven masses, etc., a delay occurs in the build-up of the pressure medium pressure so that, upon the occurrence of torque surges, the known transmissions cannot react immediately, but only with delay to the increase of load.
Moreover, it is disadvantageous that the hydraulically adjustable cone-pulley transmissions require pump pre-running or an external oil supply for the start up operation, so that firstly the requisite oil pressure and oil flow are present before the transmission is set in motion, unless a hydraulic start-up or overload coupling is provided, such as disclosed in German Pat. No. 2,256,023, which likewise again constitutes a very space-demanding, complicated and expensive component.
Moreover, the yieldability of the support of the fixedly supported cone pulley sheavers, caused by material elasticity, and the material elasticity of the fixedly supported cone pulley sheaves themselves, starting from a specific transmission ratio setting, causes a variation of transmission ratio in dependence upon the loading transmitted by the transmission. In fact, although, as a rule, this transmission ratio variation amounts to only a few percent, it can, depending on the circumstances of the utilization case, constitute a serious disadvantage.
Finally, in the known transmissions it is necessary to provide a separate oil conduit for the lubrication and cooling of the frictional contacts between the transmission means and the cone pulleys.